Two-stage apparatus for clarifying liquid charged with solid matter

ABSTRACT

A cloudy liquid to be clarified is inserted, after twice receiving flocculation additive, into the bottom of a belt of sludge (4a) in a clarification chamber (4) via a plurality of injectors (22) distributed over the area of said chamber. Uniform distribution of overall flow between these injectors is ensured by internal head loss. The clarified liquid is removed via an upper overflow (4b). Each injector is constituted by a flat cyclone and forms a jet which is highly diverting from its vertical axis in such a manner that the injected flowrate spreads out over an expansion zone belonging to said injector and meeting the expansion zones of adjacent injectors. Further, these jets set up swirling motion which prevents the formation of a layer of sludge having insufficient permeability. The sludge formed in the chamber (4) is directed via a central well (7) towards a secondary clarification stage (3) situated beneath said chamber and supplying both clarified liquid and dense sludge.

The invention relates to rapid clarification of water charged with finematerial in suspension.

Water to be clarified by the method of the invention generally occurs atthe outlet from an industrial process, and is intended, for example,either for further industrial use or else for rejection into the publicwater system. However, these indications are not restrictive in any way.Although the following description refers to practical examples in whichthe method is implemented with waste water from the minerals industry,the method may naturally be applied to clarifying liquids with a widevariety of charges.

Heretofore, generally used methods have required relatively long periodsof time to separate and sediment out solid particles in suspension andto obtain a liquid or water which is sufficiently clarified to meet thestandards laid down. This arises because the settling speed does notexceed a few meters per hour. As a result it has been necessary to useseparator equipment which is expensive, and of large size and volume.

It is known that in order to increase the rate at which solid particlessettle it is necessary to link up the particles in suspension by addinga small quantity of additive to the cloudy liquid suitable forinitiating the flocculation phenomenon. This phenomenon enables thesolid particles to come together to form agglomerates or flakes ofconsiderably greater size than the particles from which they are builtup, but having an apparent density which is less than that of the solidsfrom which they are built up.

Generally, after flocculation, the liquid and its charge of flakes ofdifferent sizes and densities is conveyed to a settling zone containinga higher concentration of flakes, thereby accelerating the increase inflake size and consequently increasing the settling speed. The flakeswhich have settled in the lower portion of the settling zone areconcentrated and they are removed (generally continuously, but sometimesin a discontinuous manner) in the form of a sludge which is more or lessconcentrated depending on a suitably determined average flow rate. Theclarified liquid is removed at the same time from the top of thesettling device from which it merely overflows. This is the case ofapparatus providing both clarification and solid settling in a singlecompartment, i.e. in a single stage.

It is also known that the two functions of clarifying the liquid andconcentrating the solid can be obtained more rapidly by separating thefunctions so that they take place in distinct parts of the apparatus.The apparatus is then said to have two stages. The stages may besuperposed with the clarification stage being above the concentrationstage, or they may be concentric, or they may be side-by-side.

However, in order to obtain the greatest effect on the speed at whichthe liquid is clarified and the solids are concentrated, it is necessaryto control all of the hydraulic movements within the various portions ofthe apparatus so as to form flakes which are as dense as possible, withthe size of the agglomerates being sufficiently high, e.g. having a sizeof 1 mm to 10 mm, and containing practically no fine material innon-flocculated suspension.

It is known that the flocculation phenomenon as initiated byflocculation additives forms agglomerates whose mechanical cohesion islow, and that hydraulic movements in the clarification zone may haveboth advantageous and disadvantageous effects simultaneously on thestructure and the physical characteristics of the agglomerates.

Speeds which are too high in some portions of the apparatus destroy theflakes already formed and form flakes having too wide a range of sizesincluding many very fine agglomerates which settle poorly. Speeds whichare too slow in other portions of the apparatus enable flakes to formwhich are too big and which agglomerate with one another. These flakesthen form continuous layers of low permeability which slow down thepassage of the liquid to be clarified. Instability phenomena then occurwith some zones being formed in which the liquid moves at high speedsthat destroy the flakes.

More particularly, a known clarification method comprises the followingoperations:

adding a flocculating additive to a cloudy liquid to cause those of saidfine particles which meet one another to bind together and form flakes;

substantially continuously inserting said mixture into a distributionchamber having substantially uniform pressure and inserting said mixtureinto a clarification chamber via a plurality of injectors distributedover the surface of said chamber in the lower part thereof;

removing clarified liquid via a liquid removal orifice in the upperportion of said chamber; and

removing sludge via a sludge-removal orifice situated further down.

The cloudy liquid injection rate is selected in such a manner as to forma belt of sludge above the injectors with the cloudy liquid risingthrough said belt between flakes which have already been formed, and thefine particles in said water binding to said flakes in order to increasethe volume thereof so that the liquid which arrives above said belt ofsludge is partially clarified and forms a belt of clarified liquid abovethe belt of sludge.

This known method is described in French Pat. No. 2 477 896 (Ghezail etal). More particularly, it includes the following characteristics:

The injectors are injection orifices through the floor of theclarification chamber. Funnel-shaped elements flare upwardly from saidorifices. In each horizontal plane passing through such a funnel, thecross-section of the passage allowed to the injected cloudy water is,according to said patent (page 5, lines 1-5), constituted by the entireinterior of the funnel, thereby very rapidly reducing the speed of saidwater after it leaves the injection orifices. This is the desired resultsince in this method it is desired above all that the belt of sludgeabove the injection orifices should be substantially free of agitation.In practice, once the liquid passes through the injection orifice at anysubstantial speed, an axial cylindrical jet is formed at a distance fromthe walls of the funnel. Further, in this method, the sludge ispartially removed on a temporary basis in a periodic manner through theinjection orifices by applying temporary suction thereto and causing theflow direction through the injection orifices to be temporarily andsuddenly reversed.

These brief periodic reversals of cloudy water flow direction prevent,to some extent, the formation of a continuous and insufficientlypermeable layer which would cause the above-mentioned instabilityphenomena to appear.

The sloping walls of the funnels serve to guide the sludge into theinjection orifices during the periodic reversals.

A second known method makes it possible to avoid, to some extent, theformation of an insufficiently permeable continuous layer of sludge.This method is described in French Pat. No. 1 115 038 (Degremont) andmakes use of periodic temporary accelerations in the flow of cloudywater through the injection orifices.

A third method is known from French Pat. No. 2 431 317 (Dorr Oliver)corresponding to U.S. Pat. No. 4,263,137 (Kos). This method uses movinginjectors including funnelshaped guide means which serve to slow downand distribute the flow of injected cloudy water.

The object of the present invention is to provide apparatus in a simpleand cheap manner for effective clarification of a belt of sludge inwhich the stability of the sludge belt makes it possible to keep theliquid rising at a high average speed.

This increased average rise speed gives increased flowrate of treatedcloudy liquid through an installation of given area, or, when treating agiven flowrate, it reduces the area required by the installation. When ahigh water flowrate is to be treated, such a reduction in the area ofthe installation constitutes a major economic advantage.

A brief description of the drawings is shown below.

FIG. 1 is a view of apparatus for performing the invention shown insection of a plane I--I of Figure 2, said plane passing through thevertical axis of the apparatus.

FIG. 2 is a horizontal section through said apparatus on a plane II--IIof FIG. 1, and on a larger scale.

FIG. 3 shows a detail of FIG. 1 to the same larger scale.

FIG. 4 is a highly diagramatic representation of the turbulence createdin the sludge belt by an injector.

The present invention provides a sludge belt clarification apparatus forcloudy liquid charged with solid particles, said apparatus being moreprecisely intended to receive a liquid charged with fine particles insuspension and to provide firstly a clarified liquid and secondly asludge, said apparatus comprising:

a clarification chamber (4);

a distribution chamber (5) extending under said clarification chamber;

means (8, 9,11) for adding a flocculation additive to said cloudy liquidin such a manner as to provide a mixture in which said fine particlesmeet one another and bind together with the aid of said additive to formflakes;

means (2) for inserting said mixture into said distribution chamberwhile setting up a substantially uniform pressure therein;

a plurality of injectors (22) distributed over the bottom area of theclarification chamber, each injector having an axis (22a) and beingprovided with guide means, and each of said injectors causing thedistribution chamber to communicate with the clarification chamber forcontinuously inserting said mixture into the bottom of saidclarification chamber with a flow which is distributed over the area ofsaid chamber;

an orifice (4b) for continuously removing clarified liquid from theupper portion of the clarification chamber (4); and

an orifice (26) for continuously removing sludge below said orifice forremoving clarified liquid;

the flowrate through said injectors being chosen in such a manner as toform a belt of sludge (4a) above said injectors through which belt amixed cloudy liquid rises between the already-formed flakes such thatthe fine particles still in suspension in said liquid bind to saidflakes to increase the volume thereof and such that the liquid whicharrives above said sludge belt is clarified and forms a layer ofclarified liquid (4a);

said apparatus being characterized by the fact that it further comprisesa thickening chamber (3) of type known per se constituting a secondarystage with said clarification chamber constituting a primary stage, saidthickening chamber being disposed below the clarification chamber andbeing co-extensive therewith; and

a central vertical well (7) having a high inlet (7a) for receiving anoverflow of flakes from the top layer of said sludge belt and forlimiting the height thereof, a low inlet constituted by at least onehole (18) in the clarification chamber (4) at the bottom thereof forreceiving large dense agglomerates of flakes formed in the sludge belt,and an intermediate height outlet into said thickening chamber (3);

said thickening chamber including an outlet for clarified water (13)higher than the outlet from said central well and an outlet for sludge(26) in the lower portion thereof.

(The reference numerals appearing in parentheses relate to the figuresand are given merely by way of example).

Further, the following dispositions are preferably adopted:

the thickening (3), distribution (5), and clarification (4) chambershave side walls which are substantially continuous, thereby facilitatingconstruction of the apparatus;

the apparatus includes an axial shaft (20) rotated by a motor (21) anddriving both a scraper arm (19) over the bottom of said clarificationchamber to bring said large agglomerates toward the low inlet (18) ofthe central well (7), and scraper arms (24) for scraping the bottom saidthickening chamber (3) to bring the thickened sludge towards said sludgeoutlet (26) from said chamber;

said central well (7) is fitted near the bottom thereof with a deflector(16) for improving the distribution of liquid having a high agglomerateconcentration within the second stage (3);

the bottom of the primary stage (4) including the injectors (22) ishorizontal or slightly conically shaped with the point being downwardlydirected;

the bottom of the secondary sludge concentration stage (3) is horizontalor slightly conical with a downwardly directed point and is providedwith rotating scraper blades (24) for continuously removing the solidmaterial settled on the bottom.

The present invention also provides a method of continuously and rapidlyclarifying a cloudy liquid charged with finely divided solid material insuspension, characterized in that the method consists in continuouslymixing said cloudy liquid with a flocculation additive injected at apredetermined dosage at at least two points (8,9) of the circuit in sucha manner as to form agglomerates or flakes of regular size and apparentdensity in a belt of sludge (4a) located in a primary clarificationstage (4), said sludge belt enabling a large portion of the liquid to beclarified to be continuously separated and removed, said portion beingup to half of the flowrate, and an enabling the concentrated cloudyliquid to penetrate into a secondary stage for clarification of theliquid and thickening of the sludge (3), and then removing firstly theclarified liquid produced by said secondary stage and secondly removingthe dense sludge deposited on the bottom thereof in continuous manner bya bottom current.

An implementation of the invention is described in greater detail belowby way of non-limiting example with reference to the accompanyingdiagrammatic figures. It must be understood, that without going beyondthe scope of the invention, the items described and shown may bereplaced by other items providing the same technical functions. When thesame item is shown in several figures, it is designated by the samereference sign in all of them.

With reference to FIG. 1, the cloudy liquid constituted by water chargedwith solid material in suspension (e.g. clay from phosphate washing) isinserted via a duct 1 into a primary flocculating tank 2 for feeding aclarifying and settling apparatus 3.

This clarifying and settling apparatus has a vertical axis 3a and ismainly constituted by two superposed and generally cylindrical stages.An upper primary stage 4 constitutes the said clarification chamber. Ithas a circular threshold 4b which constitutes the said water removalorifice over which the water overflows. Its bottom end is delimited by achamber 5 which constitutes the said distribution chamber and whichseparates the upper stage 4 from the lower secondary stage 6 whichconstitutes a thickening chamber.

The bottom of the upper primary stage 4 is shown to be horizontal inshape. It could also be conical with a downwardly directed point withoutinterfering with the operation of the apparatus. The chamber 5 locatedbetween the two stages 4 and 6 serves to feed the sludge belt 4a whichforms in the lower portion of the upper stage and within which secondaryflocculation takes place thereby forming agglomerates or flakes whichsettle and are concentrated in the lower stage 6.

The chamber 5 is fitted with injectors which are described moreaccurately below with reference to FIGS. 2 and 3.

The upper primary stage 4 and the lower secondary stage 6 arehydraulically interconnected by a central well 7 which opens out at 7ainto the thickness of the sludge belt and which serves to pass theflakes made up in said sludge belt from the primary stage into the lowersecondary stage where they settle and are concentrated.

Various commercially available flocculation additives may be used forforming the agglomerates, including various polymers, provided that theflocculating agent used is properly matched to the solid materials to besettled out and to the characteristics of the liquid to be clarified.

The flocculation additive used is diluted to the proper concentration toobtain maximum effect and it is generally injected into the apparatusvia two or three points.

As can be seen in FIG. 1, a first dose of additive is inserted at 8prior to insertion of the water to be clarified in the tank 2 in orderto trigger primary flocculation in said tank causing small flakes to beformed, having a size of less than 1 mm to 2 mm.

After this flocculation, a second dose of additive is inserted at 9 atthe inlet to the distribution chamber 5 which also serves as a mixingchamber.

Secondary flocculation takes place in the sludge belt 4a in the upperprimary stage 4. Generally these two primary flocculation stages aresufficient, but some types of particle may need a third dose of the sameadditive at a point 10 located at the bottom of the well 7 prior toentry into the lower stage 6. Sometimes the additive inserted 10 may beof a different kind since it may be an additive for accelerating theconcentration of flakes in stage 6. The various quantities of additiveinserted are measured out by a multi-head metering pump 11 or by twopumps of the same type where different additives are used.

Both stages of the apparatus provide clarified liquid: the upper stagecauses it to overflow and be removed via a circular channel 4c and apipe 12, while the lower stage 6 provides it via a pipe 13. This pipe isprovided with a valve or other hydraulic adjustment device 14 in orderto adjust the proportions of clarified liquid which leave the primarystage via the pipe 12 and the secondary stage via the pipe 13.

Generally, the flowrate of clarified liquid produced by the primarystage is practically equal to the flow rate produced by the secondarystage. This ratio may be modified if it is desired to improve thequality of the clarification of the liquid by increasing the proportionof secondary flowrate to primary flowrate, which ratio may be as greatas two.

In order to improve the stability of the sludge belt 4a in the primarystage 4, the central well 7 is fitted with a plurality of anti-vortexfins 15.

Further, in order to improve the distribution of cloudy liquid passingthrough the well 7 and feeding the lower stage 6, the bottom portion ofthe well 7 is fitted with a conically shaped deflector 16.

The well 7 is not only open at its top 7a within the thickness of thesludge belt 4a, but it is also perforated one or more holes 18 locatednear to the bottom of the upper stage 4 in order to put the bottom ofthe sludge belt into direct communication with the well. Thisdisposition allows a small portion of the cloudy liquid flowratecontained in the sludge belt 4a to be passed directly into the well 7,with the major portion overflowing the top of the well 7 prior todropping towards the stage 6. This disposition has the advantage ofcausing agglomerates which have grown too much to rise up to the top ofthe well 7 to pass directly therein.

In order to facilitate removal of large agglomerates through the holes18, slowly rotating scraper blades 19 urge these large agglomerates by acentripetal mechanical effect towards the holes into the well for theirremoval. These blades are controlled by a control shaft 20 driven by amotor and gearbox unit 21. The same shaft 20 also controls scraperblades 24 which rotate at the same speed to remove concentrated sludgefrom the bottom of the secondary stage 6 for hydraulic removal via apipe 26.

FIG. 3 is a section on a larger scale through the distribution chamber 5and its injectors, and FIG. 2 is a plan view showing some of saidinjectors.

As mentioned above, essential parameters for proper operation ofclarification apparatus are the formation of the agglomerates, theuniformity of their size, and their density.

In order to reach the desired object of the present invention, it isnecessary to distribute as uniformly as possible the flowrate of liquidto be clarified over the entire transverse area of the apparatus, i.e.the rising flow speeds through the sludge belt 4a contained in thechamber 4 should be as uniform as possible.

For this purpose, as shown in FIG. 3, the primary clarification stage 4is delimited at its bottom end by a chamber 5 whose top is constitutedby a partition 5a which constitutes the floor of the clarificationchamber 4 and which is pierced by injector holes distributed over itsarea. These holes 21 inject into said chamber the liquid to be clarifiedafter it has already received both its primary and secondary doses offlocculation additive.

Each hole 21 constitutes the outlet from a flat cyclone chamber 22 whichsets up a swirling motion so as to spread out the liquid to be clarifiedon leaving each hole 21 over an area which is 40 to 100 times greaterthan the area of each hole.

As can be seen in FIG. 2, each flat cyclone chamber 22 has a tangentialinlet 21a in order to set up swirling motion in said chamber, whichmotion continues through the holes 21 and into the bottom of the sludgebelt 4a, thereby uniformly distributing the liquid to be clarified anduniformly mixing the flocculation agents therein.

Each of these flat cylcones 22 together with its outlet hole 21 and itsinlet 21a constitutes one of said injectors, having a vertical axis 22aand setting up primary swirling motion of the type mentioned above. Thisswirling motion is represented at T1 in FIG. 4. It reaches the edge ofthe expansion zone 22b of the injector. It then returns towards the axisof said injector and a falling current appears on said axis. This setsup secondary swirling motions such as T2 thereabove and these in turnset up further swirling motion. There is thus substantial agitation inthe lower portion of the sludge belt. This agitation falls off withincreasing height, and the top layer of the sludge belt is hardlyagitated at all.

Further, as can be seen in FIG. 2, the tangential inlets to the cyclonesare placed in alternating manner with clockwise and counter-clockwiseswirling motions cancelling one another to avoid an overall swirlingmotion in the chamber 4.

In order to show the advantages of the invention and by way of example,a two-stage 1 m diameter apparatus feeding its sludge belt by means of aconventional slot system clarifies 16 to 18 m³ /n of clayey water havinga concentration of 30 to 40 grams/liter of solid materials, whereas anapparatus of the type described above in accordance with the inventionand of the same size is capable of clarifying a flowrate of 70 to 80 m³/n with the same consumption of flocculating agent per ton of settleddry matter, i.e. 90 to 100 grams of flocculating agent per metric ton.

The test apparatus was fitted with about fifty cyclone injectors forfeeding the sludge belt, each injector having an outlet hole into thesludge belt with a diameter of 20 mm.

I claim:
 1. Sludge belt clarification apparatus for receiving a cloudyliquid charged with fine solid particles in suspension to providefirstly a clarified liquid and secondly a sludge, said apparatuscomprising:a clarification chamber (4); a distribution chamber (5)extending under said clarification chamber; means (8, 9, 11) for addinga flocculation additive to said cloudy liquid in such a manner as toprovide a mixture in which said fine particles meet one another and bindtogether with the aid of said additive to form flakes; means (2) forinserting said mixture into said distribution chamber while setting up asubstantially uniform pressure therein; a plurality of injectors (22)distributed over the bottom area of the clarification chamber, eachinjector having an axis (22a) and being provided with guide means, andeach of said injectors communicating the distribution chamber with theclarification chamber for continuously inserting said mixture into thebottom of said clarification chamber with a flow which is distributedover the area of said chamber; an orifice (4b) for continuously removingclarified liquid from the upper portion of the clarification chamber(4); and an orifice (26) for continuously removing sludge below saidorifice for removing clarified liquid; the flowrate through saidinjectors being chosen in such a manner as to form a belt of sludge (4a)above said injectors through which belt a mixed cloudy liquid risesbetween the already-formed flakes such that the fine particles still insuspension in said liquid bind to said flakes to increase the volumethereof and such that the liquid which arrives above said sludge belt isclarified and forms a layer of clarified liquid (4a); the improvementwherein said apparatus further comprises a thickening chamber (3) forthickening the sludge from said sludge belt and constituting a secondarystage with said clarification chamber constituting a primary stage, saidthickening chamber being disposed below the clarification chamber andbeing co-extensive therewith; and a central vertical well (7) having ahigh inlet (7a) for receiving an overflow of flakes from the top layerof said sludge belt and for limiting the height thereof, a low inletconstituted by at least one hole (18) in the clarification chamber (4)at the bottom thereof for receiving large dense agglomerates of flakesformed in the sludge belt, and an intermediate height outlet into saidthickening chamber (3); first conveying means for conveying said largedense agglomerates from the bottom surface of said clarification chambertowards said central well low inlet; said thickening chamber includingan outlet for clarified water (13) higher than the outlet from saidcentral well, an outlet for sludge (26) in the lower portion thereof;and second conveying means for conveying thickened sludge from thebottom surface of said thickening chamber towards said sludge outlet. 2.Apparatus according to claim 1, wherein that the thickening (3),distribution (5), and clarification (4) chambers have side walls whichare substantially continuous, thereby facilitating construction of theapparatus.
 3. Apparatus according to claim 1, wherein said first andsecond conveying means include an axial shaft (20) rotated by a motor(21), scraper arms (19) fixed to said shaft and extending over thebottom of said clarification chamber to bring said large agglomeratestowards the low inlet (18) of the central well (7), and scraper arms(24) fixed to said shaft for scraping the bottom of said thickeningchamber (3) to bring the thickened sludge towards said sludge outlet(26) from said chamber.
 4. Apparatus according to claim 1, wherein saidcentral well (7) is fitted near the bottom thereof with a deflector (16)for improving the distribution of liquid having a high agglomerateconcentration within the second stage (3).
 5. Apparatus according toclaim 1, wherein the bottom of the primary stage (4) including theinjectors (22) is slightly conically shaped with the apex beingdownwardly directed.
 6. Apparatus according to claim 1, wherein thebottom of the secondary sludge concentration stage (3) is conical anddownwardly directed, and said second conveying means comprises rotatingscraper blades (24) for continuously removing the solid material settledon the bottom.